This invention relates generally to high bandwidth radio frequency receivers, and, more specifically, to radio frequency receivers wherein blind recovery of a transmitter symbol clock is necessary for reconstruction of data at that receiver.
Clock recovery of a transmitted radio frequency waveform is necessary when digitally sampling the analog waveform at a receiver. The recovered clock must be phase and frequency locked to the transmitted symbol clock, and with sufficiently low noise to reduce the likelihood of sampling the analog data at a non-ideal point in time. Clock recovery is necessary for many wireless communication systems. However, some scenarios exist which provide a challenge to adequate recovery.
When a phase-locked reference signal is transmitted alongside its digital-to-analog converted (DAC) waveform, a receiver can use the reference signal to sample the received data at the analog-to-digital converter (ADC). Often, however, a reference signal is not sent with a waveform and thus must be derived from the data itself. This type of blind recovery of a transmitted symbol clock can be challenging for simple modulation schemes such as Binary Phase Shift Keying (BPSK) and Quadrature Phase Shift Keying (QPSK), and becomes a more significant challenge for high order Quadrature Amplitude Modulation (QAM) schemes, such as 16-QAM. This is because most blind recovery schemes rely on regular zero-crossings in the data to derive the reference frequency. With high order modulation data the number of zero-crossings is potentially reduced and the recovery technique may have trouble making a decision on the clock frequency.
In addition to this challenge, many recovery schemes are designed to recover the clock of a single modulation scheme. Dynamic modulation techniques such as those with the ability to adjust to changing atmospheric or topological conditions means a recovery technique must be powerful enough to support these changes while maintaining relatively low cost and complexity.
Finally, a number of clock recovery solutions exist which rely on the ability to oversample received data and thereby reconstruct a reference clock in the digital domain. However, as signal bandwidths continue to increase the reliance on high-speed ADCs to sufficiently oversample that signal is also increasing, along with cost and complexity of the intended receiver.
An optimal solution to the blind recovery of a high order modulation symbol clock should be one that is relatively simple, low cost, and does not require oversampling of a received waveform at the ADC. The prior art has been able to isolate some of these parameters independently, but has failed to optimize for all parameters at once.
In U.S. Pat. No. 5,982,200, a Costas loop is identified to perform carrier recovery of a waveform. This provides the ability to recover a signal's center operating frequency and downconvert to a lower frequency or baseband. This Costas loop utilizes two square-law circuits to perform recovery of the carrier sinusoid. However, this operation targets carrier recovery as opposed to clock recovery, and targets QPSK as opposed to high-order modulation schemes.
In U.S. Pat. No. 5,519,733, carrier recovery of high order modulation is performed using a phase detection system with a phase lock loop. While providing detection of high order QAM, the patent uses a more complex detection system than the squaring law, and recovers the carrier of a waveform as opposed to the sinusoidal clock signal.
In U.S. Pat. No. 6,278,741, a software-based timing recovery technique is presented. While this technique is potentially powerful for low bandwidth applications where a receiver ADC can oversample a waveform to achieve more flexibility of design in the software domain, such a method is impractical for high bandwidth signals due to limitations of ADC sample rates.
In U.S. Pat. No. 5,825,825, an analog clock recovery technique is shown for recovery of the fundamental sinusoid from a baseband waveform. This process has the potential to recover a clock from a high order QAM waveform. However, the design is complex, requiring multiple digital-to-analog and analog-to-digital waveform conversions and components. Additionally, the technique requires high-speed digital components which may not support high frequency clock recovery.
In U.S. Pat. No. 7,933,362, a detection technique is presented which utilizes pilot signals and unique word synchronization for recovery. Such a method can be used efficiently, without the need for a squaring loop implementation. However, since this technique requires pilot tones, it fundamentally requires a synchronization sinusoid to be sent along with the target data. This minimizes the bandwidth dedicated to data transmission and is no longer considered a blind clock recovery detection technique. Depending on the implementation, an oversampling ADC may also be required.